Use of BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing
RFC 2545

Document Type RFC - Proposed Standard (March 1999; No errata)
Last updated 2013-03-02
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Network Working Group                                        P. Marques
Request for Comments: 2545                          cisco Systems, Inc.
Category: Standards Track                                     F. Dupont
                                                                  Inria
                                                             March 1999

  Use of BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   BGP-4 Multiprotocol Extensions [BGP-MP] defines the format of two BGP
   attributes (MP_REACH_NLRI and MP_UNREACH_NLRI) that can be used to
   announce and withdraw the announcement of reachability information.
   This document defines how compliant systems should make use of those
   attributes for the purpose of conveying IPv6 routing information.

1. Introduction

   The BGP-4 protocol [BGP-4] in particular, and path vector routing
   protocols in general, are mostly independent of the particular
   Address Family for which the protocol is being used.

   IPv6 falls under the generic category of protocols for which BGP-4 is
   suitable and, unless stated otherwise in this document, the BGP-4
   procedures to apply when using BGP-4 to carry IPv6 reachability
   information are those defined in [BGP-4] and in subsequent documents
   that extend or update the BGP-4 specification.

   In terms of routing information, the most significant difference
   between IPv6 and IPv4 (for which BGP was originally designed) is the
   fact that IPv6 introduces scoped unicast addresses and defines
   particular situations when a particular address scope must be used.
   This document concerns itself essentially with the necessary rules to
   accommodate IPv6 address scope requirements.

Marques & Dupont            Standards Track                     [Page 1]
RFC 2545      BGP-4 Multiprotocol Extensions for IPv6 IDR     March 1999

2. IPv6 Address Scopes

   IPv6 defines 3 unicast address scopes [ADDR-ARCH]: global, site-local
   and link-local. Site-local addresses are non-link-local address which
   are valid within the scope of a "site" and cannot be exported beyond
   it. As this document makes no assumption on the characteristics of a
   particular routing realm where BGP-4 is used, it makes no distinction
   between global and site-local addresses and refers to both as
   "global" or "non-link-local". Network administrators must however
   respect address scope restrictions and should be aware that the
   concepts of a BGP-4 routing domain and "site" are orthogonal notions
   and that they may or may not coincide in a given situation.

   Companion IPv6 specifications further define that only link-local
   address can be used when generating ICMP Redirect Messages [ND] and
   as next hop addresses in some routing protocols (eg. RIPng [RIP]).

   This restrictions does imply that an IPv6 router must have a link-
   local next hop address for all directly connected routes (routes for
   which the given router and the next hop router share a common subnet
   prefix).

   Link-local addresses are not, however, well suited to be used as next
   hop attributes in BGP-4 given the rules defined for this attribute in
   the protocol specification [BGP-4].

   For the above reasons, when BGP-4 is used to convey IPv6 reachability
   information it is sometimes necessary to announce a next hop
   attribute that consists of a global address and a link-local address.
   The following section describes the rules that should be followed
   when constructing the Network Address of Next Hop field of an
   MP_REACH_NLRI attribute.

3. Constructing the Next Hop field

   A BGP speaker shall advertise to its peer in the Network Address of
   Next Hop field the global IPv6 address of the next hop, potentially
   followed by the link-local IPv6 address of the next hop.

   The value of the Length of Next Hop Network Address field on a
   MP_REACH_NLRI attribute shall be set to 16, when only a global
   address is present, or 32 if a link-local address is also included in
   the Next Hop field.

   The link-local address shall be included in the Next Hop field if and
   only if the BGP speaker shares a common subnet with the entity
   identified by the global IPv6 address carried in the Network Address
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